In general iPXE files are licensed under the GPL. For historical
reasons, individual files may contain their own licence declarations.
Most builds of iPXE do not contain all iPXE code (in particular, most
builds will include only one driver), and so the overall licence can
vary depending on what target you are building.
The resultant applicable licence(s) for any particular build can be
determined by using "make bin/xxxxxxx.yyy.licence"; for example:
make bin/rtl8139.rom.licence
to determine the resultant licence(s) for the build bin/rtl8139.rom